Doxygen updates

- tls hooks support: special tls hooks added in core that allow a module or core part to provide a tls implementation. Support for the old in-core tls compile options is preserved (but one can compile with either tls hooks support or tls-in-core support, not with both in the same time). Changes were kept to a minimum. - Makefiles: make TLS=1 deprecated, use instead make TLS_HOOKS=1 or make CORE_TLS=1 (if in-core tls support is needed). Added TLS_EXTRA_LIBS (e.g. make CORE_TLS=1 TLS_EXTRA_LIBS="-lz -lkrb5").